L&T: Lens Choices

Jul2009

Winning the High-Index Numbers Game

High-index plastic lenses can greatly improve the eyeglasses
you build. Using a high-index lens material—one with an index of
refraction ranging from 1.60 to 1.74—can reduce lens thickness and
weight, making eyeglasses comfortable and cosmetically appealing.
High-index lenses can also be used to improve the fit of lenses in the
frame.

Because of the ever-expanding range of high-index lenses on the
market, many dispensers are unsure of how choose an appropriate lens
for a patient. With so many lens materials and lens treatment
combinations available, knowing when to recommend one high-index lens
versus another can seem like a numbers game. To win that numbers game,
you’ve got to master a few key optical concepts and facts.

Understanding
Index of Refraction
Index of Refraction is the speed of light in a vacuum divided
by the speed of light in a given material. The resulting number (1.5,
1.6, 1.67, 1.74) is referred to as the “index.” The higher the number,
the more light is slowed down or bent. This means high-index lenses
bend light at steeper angles than standard-index, hard resin lenses.
Therefore, high-index lenses need less curvature to achieve the same
focal power. That’s why a lens made from a 1.67-index material can be
significantly thinner than a standard-index (1.50) plastic material
with the same prescription.

An additional benefit of high-index lenses over standard plastic
lenses is that they offer inherent UV protection as opposed to having
to apply a UV coating, which may cause some standard-index lenses to
turn yellow.

High-index lenses also have flatter curvatures than their standard
plastic cousins. Aspheric design, when combined with high-index resin,
will further thin and flatten both plus and minus powers. All these
factors make high-index lenses an especially good match for today’s
frames, which are of an increasingly flat design. And its high tensile
strength makes a 1.67 material a good choice for drilled rimless.

Factoring in Abbe Value
When looking through a lens at a point other than the optical
center, the component colors making up white light are displaced
laterally by differing amounts resulting in a degraded image, so a
challenge in high-index lens design is minimizing effects of chromatic
aberration, or color dispersion occasionally noticed by the eyeglass
wearer as a “halo” or color fringes around the edges of an object.

A scale used to rate the degree of chromatic aberration of optical
materials is the Abbe value. A lower Abbe value indicates more
chromatic aberration and a higher Abbe value indicates less.

Comparing High-Index Materials

Material

Refractive Index

Abbe

1.74

1.74

32

1.70

1.70

36

1.67

1.67

32

1.60

1.60

42

Polycarbonate

1.59

32

Trivex

1.53

45

Standard Plastic

1.50

58

A “sweet spot” exists at and around the optical center of
lenses within which chromatic aberration is more tolerable. Minimal
decentration of lenses in the frame and a smaller ED minimizes the
effect of chromatic aberration. The idea is to position the best optics
directly in front of the patient’s pupil and to reduce areas of the
lenses in which the chromatic aberration is most noticeable. There is
no practical way to eliminate chromatic aberration, but anti-reflective
(AR) lenses can help patients tolerate the problem.

The AR Advantage
When dispensing high-index materials, AR lenses offer
significant advantages over non-AR lenses. Because high-index materials
are relatively dense, they have significantly higher reflectance than
lower index lenses. This higher reflectance creates increased veiling
glare under ordinary lighting and also creates annoying ghost images
when bright light sources are introduced into an environment of
generally low illumination (e.g. oncoming headlights during night
driving or candles in a darkened restaurant). By reducing the higher
reflectance that is otherwise going to be present whenever index is
increased, the optical performance and the cosmetic appearance of these
lenses are both improved.

Presenting High-Index Options
When recommending a high-index lens to a patient, remember to
go easy on the technical features. Focus instead on the benefits the
patient will receive (while always being prepared for questions and
concerns). You’ll feel more like the expert when you act like the
expert, recommending what’s best and keeping in mind the individual’s
prescription and lifestyle needs, rather than presenting a laundry list
of every option.

Always consult with your optical laboratory or lens sales
consultant if a patient is having difficulty wearing a particular lens
material. They can recommend other lens materials and treatments. For
example, a patient who cannot tolerate a polycarbonate material or a
high-index can still be given a highly impact-resistant material by
using a lens made of PPG’s Trivex, which features a 1.53 index and an
Abbe value of 44.

With knowledge, ability and a little patience, it’s possible to
ascertain what your customer wants and then combine several elements of
cosmetics and technology into a pair of glasses that are as attractive
as they are functional.

Timothy Coronis ABOC-NCLE, is a certified
optician and contact lens examiner based in Keene, N.H. He is also an
American Board of Opticianry technical speaker. He may be reached at
timothycoronis@hotmail.com.

Solving Patient NeedsHere are some examples of how choosing the right high-index material
can solve a variety of patient’s optical and cosmetic needs.

Example 1 The patient’s prescription is -8.00 R –7.75 L and the PD is 58mm.
The patient selected a fashionable, rectangular frame, 52-20, and is
asking about polycarbonate lenses.

This prescription/lens/frame combination could spell trouble. The
fact that the lenses are decentered 7mm each and rectangular shape
means the outer edges of the lenses would be very thick. It’s better to
avoid thick lens edges through careful frame selection and choice of
materials whenever possible.

A useful tip when building glasses and anticipating how lenses
will look in the frame is to remember that greater decentration causes
thick lateral edges for minus prescriptions and thicker centers for
plus lenses. The former case is exactly what we’re trying to avoid.

Solution Recommending a smaller, 48-18 as something to complement the
lenses, will result in a decentration of only 4mm each. A less severe
frame shape such as an oval means there will be less thick lens edge in
the eyeglasses. These decisions, combined with a 1.74 index and an
aspheric design, mean there will be much thinner lenses in the frame.
In our new example, each factor worked toward the same goal, thinner,
more attractive lenses.

Example 2 The patient’s prescription is +0.75 OD, +4.25 OS, PD is 65. The
patient picked out a 48-20 zyl frame and then had eye exam. These are
two very different lenses and two very different powers. Glazing a zyl
frame with these lenses would result in the OS temple being angled
inward because the left eyewire must be molded around the steep lens
curvature, while the OD side would be more nearly at right angles to
the front.

Solution To achieve better symmetry and to get this prescription into a zyl
frame in an artful and practical manner, the optician suggested a 1.67
aspheric lens for the OS and a standard plastic, non-aspheric for the
OD. The high-index makes the OS lens thinner and being aspheric will
makes it even flatter. The OS lens is a non-aspheric standard plastic.
In spite of dissimilar powers, the lenses will look like a matched pair
and will behave similarly in the frame, and the patient with dissimilar
eyes can receive eyeglasses of the same cosmetic standard as anyone
else. AR lenses are also recommended for low power lenses.

Patients with this much anisometropia will be more prone to
spatial distortions if they have approximately equal acuity between the
eyes, and moderate to good stereopsis. A consultation with the Rx-ing
doctor would be a good idea before proceeding.

Example 3 The prescription is +1.00 OD and +1.25 OS, add power is +2.75 OU
and the PD is 56. The patient likes a grooved and rimless frame design
that measures 52/18. He wants to use his glasses for reading only and
he likes the idea of standard plastic being the cheapest option. He
mentions having gotten “eyeglasses-in-an-hour” last time.

This is a different type of potential problem. The full reading
prescription here is +3.75 OD and +4.00 OS. Plus lenses are thickest in
the center and thinnest at the edges. The patient’s PD of 56mm means
the lenses would have to be decentered 7mm each, which would result in
eyeglasses with lenses inordinately thick both near the bridge and in
the centers. In fact, in this particular case, ordering lenses surfaced
to get sufficient thickness for grooving at the temporal edge would
make the overall lens thickness considerably greater.

After reflecting on similar situations, the dispenser explained to
the patient that in order to get the lenses in their best form, another
frame would have to be selected.

Solution Use 1.67 aspheric lenses mounted in a zyl frame to conceal the
thick nasal edge and a rounder frame design at 48/19 to reduce
decentration. The lens and frame combination will harmonize quite well.
The aspheric greatly reduces the lens thickness at the nasal edges.

Example 4 The prescription is -4.00 OD, -3.75 OS. Patient likes drilled
rimless frames. Conversation revealed the patient works in a bank, uses
a computer screen all day and deals with the public. Recommendations
should include that AR lenses present a nice appearance, relieve
eyestrain from prolonged computer use and improve the appearance of
rimless designs.

Solution Rimless, eight-hole drilled, anti-reflective lenses for looking at
the computer screen all day, fabricated with 1.67 drillable material
with a high-tensile strength.